(1) Field of the Invention
The present invention relates to the field of rotorcraft having a power plant comprising a plurality of engines, which engines are used selectively for driving at least one rotary wing of the rotorcraft in rotation.
The present invention relates more particularly to rotorcraft arrangements relating to feeding such a power plant with fuel. A particular object of the present invention is to provide an architecture for feeding fuel to a power plant fitted to a rotorcraft, which power plant has a plurality of engines, and in particular two engines, and serves to drive the rotary wing in rotation selectively from one and/or the other of the engines.
(2) Description of Related Art
Rotorcraft are aircraft in which lift, and possibly also propulsion and maneuvering in flight, are obtained by means of at least one rotary wing forming part of a rotorcraft. The rotary wing is driven in rotation at a speed that is generally constant by a power plant, and it comprises blades that can be operated by actuators in order to vary their pitch collectively and/or cyclically. Varying the pitch of the blades serves to provide the rotorcraft with propulsion and/or maneuvering in flight. The rotary wing may equally well be a main rotary wing providing at least the lift if not also the propulsion of the rotorcraft, a propulsive propeller in a hybrid helicopter, for example, or a rotary tail wing that serves to provide yaw maneuvering for the rotorcraft. In order to limit the number of on-board engines included in the power plant, it is common practice to use a single power plant to drive the various rotary wings of the rotorcraft in rotation.
Rotorcraft are commonly arranged in various categories, depending on the architecture of their power plants and more particularly as a function of the safety provided by the power plant when faced with a possible failure. One distinction between two categories of rotorcraft is associated in particular with their ability to fly safely over various kinds of territory, especially in the event of a possible failure of the power plant.
By way of example, rotorcraft of category A are rotorcraft in which the power plant has a plurality of engines, and in particular two engines, which engines serve to drive at least the main rotary wing. In the event of a first engine failing, a second engine must be capable of being used to enable the rotorcraft to continue flying, possibly so as to move away from sensitive territory, such as a densely populated area, for example.
Rotorcraft of category A should be distinguished in particular from rotorcraft of category B of organization that does not satisfy such requirements for safely overflying sensitive territory. Rotorcraft of category B may have one or more engines, but their ability to continue flying in the event of an engine failure does not comply with constraints relating to satisfying regulations for overflying sensitive territories.
With rotorcraft of category A, there arises a problem of organizing how to feed fuel to the various engines making up the power plant. Account must be taken of the constraints to which rotorcraft of category A are subjected for overflying sensitive territory. The architecture for feeding fuel to the power plant fitted to a rotorcraft of category A is more complicated than for rotorcraft of category B, since each engine of the power plant must be capable of being fed with fuel for a determined duration in the event of the other engine failing.
Traditionally, the fuel feed architecture of a power plant fitted to a rotorcraft of category A has fuel feed assemblies that are allocated respectively to each of the engines in order to enable them to operate selectively and independently.
Each assembly comprises a fuel tank and a safe tank containing some minimum quantity of fuel.
The fuel tank may have a single compartment or it may be constituted by a plurality of compartments that are in free fluid-flow communication with one another. Such free fluid-flow communication naturally achieves spontaneous balancing due to gravity one with the other of the compartments, for the quantity of fuel contained in each of them. The safe tank is formed by an enclosure that is independent from free fluid-flow communication of fuel, at least from the fuel tank to the safe tank.
The safe tank has a capacity that must enable the corresponding engine to be supplied with a safe quantity of fuel. The safe quantity of fuel corresponds to supplying the engine with sufficient fuel to ensure that in the event of a rotorcraft failure it can continue to fly for some minimum length of time that is set by regulations. As an indication, such a duration for continued flying is of the order of twenty minutes, corresponding to the rotorcraft being able to fly far enough away from the territory that is considered to be sensitive.
The engine is in fluid-flow communication with the safe tank included in the assembly allocated thereto via a circuit for supplying fuel from the safe tank to the engine. The supply circuit includes a pair of booster pumps that dip into the safe tank, one of the booster pumps taking over from the other booster pump in the event of a failure. In order to guarantee that the engine is supplied with fuel in the event of both booster pumps failing, the supply circuit also includes a supply pump that is driven by the engine.
Each safe tank is fed with fuel from the fuel tank of the corresponding assembly via a feed circuit that includes feed pumps housed inside the safe tank. It is necessary to make the feeding of fuel to the safe tank secure, and, as for the booster pumps, the feed pumps are two in number, with one taking over from the other in the event of a failure.
In order to avoid redundancy of the pumps located inside the safe tank, it is common practice to use the booster pumps to act as the feed pumps. The feed circuit includes one ejector per compartment for trapping fuel and delivering it to the corresponding safe tank. The ejector, or an analogous member for capturing and delivering a fluid inside a fluid flow circuit, is an item that is common in the field of feeding fuel to a power plant fitted to a rotorcraft. From a flow of fluid inside the feed circuit, which flow is in particular forced by one of the booster pumps forming a feed pump, the ejector causes the fuel contained in the corresponding compartment to be delivered to the safe tank.
Since the capacity of the safe tank is limited, although not less than that which is necessary for continued flight in the event of a failure of the rotorcraft, the safe tank is fitted with an excess fuel device that returns the excess fuel from the safe tank by overflow to the fuel tank.
The overall supply of fuel on board the rotorcraft is stored inside the tanks of the various assemblies. A fuel transfer circuit is interposed between the fuel tanks and includes a transfer pump that can rotate in both directions and that causes fuel to flow selectively from either one of the fuel tanks to the other. Fuel is transferred from either fuel tank towards the other, being delivered to the bases of the safe tanks.
The transfer circuit enables the quantity of fuel to be balanced from either one of the circuits to the other. Such balancing is useful for distributing the weight of fuel on the rotorcraft. The transfer circuit also makes it possible, in particular in the event of one of the engines failing, to make use of all of the on-board fuel that can be consumed. The safe tanks are kept fed so long as all of the on-board fuel has not been consumed.
An intercommunication circuit is also provided that is arranged between the fuel tanks and that is arranged as a spillway. The intercommunication circuit is in free fluid-flow communication with each of the fuel tanks, in their top portions relative to gravity, so as to allow fuel to escape from either one of the fuel tanks to the other in the event of there being excess fuel in one of them.
In the field of aviation, airplanes are also subjected to the constraints of aircraft of category A relating to the ability to continue flying for a determined duration, as mentioned above. The arrangement of an architecture for feeding fuel to the engines of an aircraft is closely linked with the specific organization of the aircraft. By way of example, such an architecture must take account of the ways in which the aircraft is supported and propelled in flight, and of the surroundings available for receiving and interconnecting the various members and fuel-conveying circuits that are included in the architecture.
Nevertheless, in order to refer to a technological environment relating to the structural members included in an architecture for feeding fuel in aircraft, reference may be made for example to the following documents: US 2010/051749 (Tanner R. B.); U.S. Pat. No. 3,275,061 (Williams R. L. et al.); FR 2 623 774 (Aerospatiale); EP 2 074 027 (Boeing Co.); or EP 0 670 264 (Daimler Benz Aerospace AG, Tupolev AG); or indeed to the following document U.S. Pat. No. 3,275,061 (Williams R. L. et al.). Reference may also be made to document WO 2009/139801 (Sikorsky Aircraft Corp.) which describes a fuel feed architecture for a rotorcraft having a power plant including a pair of engines.
It is found that the conventional architectures for feeding fuel to the power plant of rotorcraft of category A would benefit from being simplified. Such simplification would need to be obtained without losing sight of the constraints to which rotorcraft of category A are subjected, in particular with respect to rules concerning continuing flight in the event of one of the engines of the power plant failing.